In issue 3/2022 of Geotechnik, Prof. Ziegler and his former doctoral graduate Dr. Ing. Dipl.-Wirt.-Ing. Elias Tafur published an essay entitled "Harmonization in geotechnics - a statistical method for determining the representative values of geotechnical parameters taking into account prior information". It should be seen as a follow-up to the paper "Harmonization in geotechnical engineering - a unified verification method", which was published last year.
The representative values are the values that are used in the limit state equation in the safety analysis. Their determination is usually made by a geotechnical expert, who sets the values as „cautiously estimated mean values" on the basis of laboratory and field tests as well as on the basis of his experience. However, the new Eurocode 7-1 equally allows the determination on the basis of statistical methods, whereas the choice of the determination method is not mandatory. Annex A merely describes a possible method for determining the characteristic value. Here, the representative value xrep results from the characteristic value xk, which may be multiplied by a conversion factor η, which stands for different effects e.g. humidity or temperature, but is often set to 1.0.
A common objection to the use of statistical methods is that the usually small number of available outcrops is not sufficient to form a solid database for the application of statistical methods. As a result, a characteristic value is obtained for which such a wide spread has to be taken into account that the output cannot be used in a reasonable way. The situation is considerably improved if additional preliminary information on the soil is available and is taken into account in the statistical evaluation. This information can be obtained from previous soil investigations in the vicinity or from generally confirmed geological findings on the surrounding soil. If such preliminary information is available, useful values can be determined even with only a few outcrops. In this paper, a method is presented that specifies the procedure according to EC 7-1 with regard to the open issues. Depending on the quality of the preliminary information, the procedure can also be used to specify the minimum number of outcrops required to achieve a specified reliability level for the representative values determined using this method. For this purpose, a reliability index β of 3.3 for a reference period of 50 years was chosen in this paper.